1. Field of the Invention
This invention relates to non-light-emitting or passive displays, and, more particularly, to means and apparatus for lighting such passive or non-light-emitting displays.
2. Description of the Prior Art
As non-light-emitting or passive displays, such as liquid crystal displays, electrophoretic and electrochromic displays, increase in popularity and usage, considerations have been given to lighting such displays, especially for nighttime use, or for use in surroundings which are not well-lit. With particular reference to liquid crystal displays, such displays typically modulate light which is incident upon it. One limitation of popularly used twisted nematic liquid crystal displays is that the contrast ratio between an actuated segment and the display background varies, depending upon the angle from which the display is observed. The limitation in inherent, and is due to the optical properties of the liquid crystal material and polarizers which are part of the display. This variation is exhibited when light is provided from either the front or the rear of the display. However, the viewing angle limitation for such device is much more severe for the back-lighted condition in which light passes through the display only once; whereas, for the front-lighted condition, light passes through twice.
In certain environments, such as in an airplane cockpit, for example, an as-wide-as-possible viewing angle is desired (as contrasted, for example, from an ordinary watch display in which the viewing angle need not be particularly extensive). Thus, it is desirable to front-light displays of the twisted nematic liquid crystal display type, both to achieve a higher contrast ratio and to increase the viewing angle.
It is known in the art to light electromechanical displays and such with a transparent planar surface overlying the display, the surface having a wedge-shaped cross-section for light distribution. The light from the light source is usually injected at the large end of the wedge and allowed to bounce back-and-forth until the critical angle of the wedge is exceeded. The light then escapes the wedge to fall upon and light the underlying display. In some instances, an inverted wedge has also been used in combination with a lighting wedge, as above described, to reduce stray light. Spherical wedges, lenses which can be considered a combination of a lighting wedge and an inverted wedge, have also been used to light displays. In such cases, light is injected in one portion of the wedge and it is reflected and redirected in the inverted portion onto the display.